Adhesive materials which are melted and liquified just prior to their application on a substrate are referred to as "hot melt" adhesives. Hot melt adhesives are available in bulk, in such forms as chicklets, pellets, slats and bricks and are also available in the form of a slug held in a container, such as a can, pail, or drum. A drum of bulk adhesive material may be as large as a conventional 55 gallon drum for large scale applications. The present invention is directed at solving various problems encountered with melting materials such as hot melt adhesives.
Often, hopper-type dispensers are used in the liquifying of hot melt adhesives. In the case of hot melt adhesive supplied as a slug in a container, it is desirous to cause the slug of adhesive to be removed from the container so that it may be efficiently melted when it comes in contact with a heated melting grid at the bottom of the hopper. A heated reservoir disposed beneath the melting grid receives the melted or liquified hot melt adhesive as it passes through the grid and maintains it in a heated liquid state. An outlet of the reservoir typically leads to a pump for pumping the liquid hot melt adhesive to other equipment, such as apparatus for applying material to substrates.
Prior hopper-type dispensers typically have one or more heaters disposed about the hopper. The heater or heaters may be attached to the side wall of the hopper to heat a container supported therein. To loosen a slug of adhesive from a container, adhesive along the inside surfaces of the container walls must be softened or melted to the extent that gravitational force on the adhesive slug can overcome the adhesive or frictional forces which tend to bind the adhesive to the walls and bottom of the container. When sufficient heating has occurred, the slug of adhesive will break loose from the walls of the drum, slide out of the drum and come into contact with the melting grid where the adhesive slug can be divided up into small portions and efficiently melted.
After the slug of hot melt adhesive has moved downward and partially out of the container such that a lower end thereof rests on the melting grid, it is generally desirable to remove the container from the hopper. In some cases, if the container is not promptly removed from the hopper, the molten hot melt adhesive may flow upward between the outside surface of the container and the hopper. This backed up hot melt adhesive sticks to the outside of the container and impedes handling and disposal of the container. In present hot melt adhesive systems, it is difficult to detect when the hot melt adhesive has moved out of the container opening onto the melting grid.
U.S. Pat. No. 4,919,308 issued in the name of Majkrzak discloses one known adhesive melting device and recognizes the problem of having liquid adhesive back up around the sides of the drum or container. However, the Majkrzak patent merely states that heat should be applied such that the hot melt material is released within fifteen to twenty minutes. The Majkrzak patent fails to disclose any sure manner of informing the operator that the material has in fact been released such that the container may be removed to prevent adhesive back up around the container.
U.S. Pat. No. 2,544,661 issued in the name of Fossa discloses another earlier adhesive melting device which includes adjacent adhesive chambers. One chamber holds a container of adhesive and melts the adhesive from the container, while the adjacent chamber holds a quantity of adhesive and dispenses that adhesive as necessary. The adjacent chamber includes a float which indicates the level of adhesive therein and includes a collar which indicates when the supply of adhesive within the container has been completely exhausted. This system, however, apparently only detects when the adhesive in the container has been completely exhausted from the container.
Accordingly, there is a need for a device and method for more precisely detecting when thermoplastic material, such as a slug of hot melt adhesive, has initially moved out of a heated container so that the container may be removed from the device. This detection would also provide a way to initiate desired control actions which depend on the initial movement of the material from the container.